Carbonate-containing raw materials for the production of calcined or sintered products, especially cement clinker, are subjected to calcination or firing in a rotary kiln, downstream of which can be provided a cooler for the sintered clinker and upstream of which there can be disposed a suspension heat exchanger for a preheating and/or precalcination of the crude solid (powder) before it is introduced into the rotary furnace.
The major portion of the fuel used in the system can be provided for the precalcination and preheating operations since a substantial portion of the deacidification, i.e. removal of carbon dioxide from the carbonates, can be effected in the precalcination stage. The combustion air required in the precalcination stage may be obtained in part from the air heated in the cooler, i.e. the air used for cooling the cement clinker in a grate-type or like cooling unit downstream of the rotary kiln.
In the upstream suspension gas preheater, the solid (powder) to be charged into the rotary kiln is subjected to preheating by intimate contact with waste gases from the rotary kiln. The fuel supplied to the preheating and precalcination stages serves to effect the additional heating necessary for deacidification which is practically complete as the solids leave the precalcination stage, thereby improving the throughput of the rotary kiln.
Because of the endothermic deacidification of the calcium carbonate in the precalcination and suspension preheater upstream of the rotary kiln, in which heat exchange takes place between the solid particles and the suspending gas with a higher heat transfer efficiency than can be maintained in the rotary furnace, it is possible to increase the throughput of a rotary furnace from four to five times above that which would obtain in the absence of such precalcination and deacidification upstream of the rotary kiln. This is discussed in German patent document (Published Application or Auslegeschrift) DE-AS 22 62 213.
It is considered important in this system to heat the hot gases used in heat exchange in the suspension gas preheater by supplying fuel to cooler-heated air and effect a combustion of the fuel with an oxygen-containing gas which has thus been relatively enriched thermally by preheating. Reference is made, in this connection, to German patent document (Open Application or Offenlegungsschrift) DE-AS 2 361 066 and German patent document (Published Application-Auslegeschrift) DE-AS 2 356 740.
The principle of repetitively reheating the carrier gas and the use of the cooling air is described in TONINDUSTRIE ZEITUNG, 1965, No. 15/16, pages 351-365.
The significance of the sequence, nature and manner in which the additional fuel and the combustion air for the burning thereof are supplied to the precalcination system is clear from the above mentioned references and the references discussed below. The parameters and conditions differ in each case and there are many theories as to why any particular treatment of the problem should be considered the correct one. In general, however, it can be said that the various solutions known to date or proposed to date are contradictory or mutually exclusive or mutually inconsistent.
In German patent document (Open Application-Offenlegungsschrift) DE-AS 2 361 006 it is proposed to use a portion of the fuel to heat the prewarmed gas (air) from the cooler and then to mix it with the furnace exhaust gas and, in a mixing chamber in which the raw powder is circulated, to introduce the remainder of the fuel through a burner to effect the main heating.
In German patent document (Printed Application-Auslegeschrift) DE-AS 2 247 172 it is proposed to burn the fuel in a separate combustion chamber to which the cooler air is supplied while the crude powder is circulated between this combustion chamber and the lowest cyclone stage of the suspension gas preheater.
In both cases it is found that the concentrated introduction of fuel leads to local overheating in the oxygen-rich suspension of the powdered material in the furnace exhaust gas because, on the one hand, of the injection of the fuel in the manner described and, on the other hand, because combustion air is mixed with the suspension before the fuel is introduced.
The result of such local overheating is the fusion of some of the particles into larger bodies, the melting of particles into droplets, the plugging of portions of the flow cross section, the undesired deposition of materials from the solids and, in general, thermally inefficient operation.
That this problem has been recognized in the art is clear from German patent document (Open Application-Offenlegungsschrift DE-AS 2 307 027) in which it is proposed to provide an intimate mixture of fuel and the crude or raw powder before this mixture is blown into the oxygen-containing hot gas.
While the latter publication does not describe how to obtain a uniform distribution of the comparatively small quantity of fuel on the relatively large quantity of solids, efforts to bring about the mixture of a liquid fuel which amounts to 3% to 4% by weight of the solids, with the raw meal of high temperature (500.degree. C.) resulted in nonuniform distribution even when energy-consuming blending at high apparatus cost was adopted. In fact, attempts to follow the teaching of the last-mentioned publication also resulted in local concentrations and hence the localized over-heating and melting or fusion characteristics and the baking of solids onto surfaces of the precalcination stage so that plugging occurs.